Significance To characterize the activities of the contraceptive NORPLANT. Objectives Four monkeys received two capsules each of NORPLANT containing 70 mg of Levonorgestrel in silastic rods. The capsules were inserted subdermally in a "V" pattern to the inter-scapular area. Blood samples were obtained once prior to treatment, daily from days 1 through 7, once per week to day 896, then biweekly to day 1888. Additional blood samples were collected beyond this time-point on a bi-weekly schedule out to day 1888. All sera was analyzed for serum levonorgestrel (LNG) by radio-immunoassay. Initially, the implantation site was also observed for any localized reaction to the capsules, then palpated periodically to check for any migration or diminishing size of the capsules. Animals[unreadable] menstrual cycles were monitored for any changes in frequency, absence and duration of cycle. Results The remaining three animals showed levels of LNG remaining fairly constant at 1.26 ng/ml (SEM 0.27) on day 1566 and 1.87 ng/ml (SEM 0.53) on day 1608, with standard deviation of 0.33. After 1/23/97 (day 1608), one animal died from pancreatopathy, hepatic lipidosis and renal lipidosis, and the remaining two animals continued to have biweekly blood collections. These two had levels of circulating LNG ranging from 1.4 ng/ml (SEM 0.38) and 2.24 ng/ml (SEM 0.17) during 2/6/97 through 6/26/97. After 6/26/97 (day 1762), one animal was euthanized due to pancreatopathy, hepatic lipidosis and renal lipidosis. The remaining animal continued on bi-weekly blood collections. The levels of LNG from 7/10/97 (day 1776) to 10/30/97 (day 1888), ranged from 1.28 ng/ml to 1.93 ng/ml. The menstrual calendars have shown amenorrhea for two of the animals, and a single light bleeding episode for one of the animals. Future Directions Collection of blood may continue until LNG drops below contraceptive effectiveness, 0.3 ng/ml. Menstrual calendar observations will continue. KEYWORDS levonorgestrel, NORPLANT., steroidal contraception, subdermal implants, synthetic progestin